Carrier wave system



2 Sheets-Sheet 1 C. L. WEIS, JR

CARRIER WAVE SYSTEM Filed Nov 2,

/NvE/v TOR C`. L. WE/.S JR. @VJ

A T TURN/5 yv April 21, 193s.

April my 19%,. C. L. wE-lmn 038,202

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` soo Ywo soo HUM Patented Apr. 21,l 19536 CARRIER WAVE' SYSTEM Charles Il. Weis, Jr., Long 'Island City, N. Y.,

assigner to Bell Telephone' laboratories, In-

corporated, New York New York N.; Y., a corporation of Application November 2, 1933, Serial No. 696,319

` 'z claims. (ol. 179-15) The present invention relates to a multiplex carrier wave transmission system and more particularly to a means of diverting from or adding to the system at an intermediate point of the system a portion only of the total number of carrier waves or channelsof the system.

An object of the invention is to permit the taking from or adding to the system at an intermediate point of the system a portion only of 10 the total number of carrier waves'or channels of the `system at the same time permitting normal transmission through the remaining channels of the system.

In a multiplex carrier-wave transmission system the channels must be placed as close togetheras possible in frequency if the mostvefficient -use'of the available vfrequency range is to be made. Complicated modulating and demodulatlng systems employing very complex systems of frequency selectivel devices have sometimes\proven advantageous economically as aids in achieving this end. AWith the advent of improved transmission mediums, such as the-concentric conductor and carrier cablesand of improved carrier apparatus the usable frequency range for wire transmission has been in some cases greatly extended.

A definite trend toward increasing the number of channels per system gives the problem of terminating or originating channelsat intermediate points along the system an increased'importance.

To solve this problem it is necessary to provide a means of effectively separating the chan- Vnels that are to be diverted, or added, from the through channels without:l interfering with normal transmission over the latter. It is also necessary to provide a means of isolating effectively one portion of the transmission system from the other at the, frequencies utilized by theA channels which are to be diverted, or added, at the same time permitting normal transmission over the' through channels.

The invention provides a solution of the above problems and makes possible the transmission convenient intermediate points the main trans- E placed in the line at a point at which channels.

are to be diverted from or added to the system. 'I'his filter has a suppressionl range corresponding to the frequencies included in the channels that are to be diverted from "or added to the system. It therefore effectively severs the line into` two portions isolated from each other with respect to these particular frequencies but allows free i transmission oft other frequencies over, the line. .Band-pass filters passing the frequencies suppressed by the band elimination filter are then connected to the line at each side of the band velimination filter. One of these serves to divert channels from the line to suitable lcarrier receiving apparatus and the other to add channels to those on the line from suitable ,carrier transmitting apparatus. The receiving and transmitting apparatus may be at the intermediate lpoint or be connected to a distant point by a suitable carrier transmission line. In accordancewith the invention, a filter of n, the piezo-crystal type may be use d as the band elimination filter at reasonably high frequencies, forexample, up to the order of 1000 kilocycles. At higher frequencies the problem becomes more difficult and-it is found necessary or ladvantageous to supplement the fllter with other circuitsy in order to obtain effective separation between the desired channels-without introducingr distortion into the through channels. In accordance with a further feature of the invention therefore, circuits vare shunted around the band elimination filter to produce a more complete separation between the waves of the channels that are to be taken from the line or added to it and the adjacent through channels and to compensate for the band edge eifects on adjacent channels of the band elimination filter. The necessary degree of `separation may be obtained in these shunt circuits by demodulating to a much lower frequency band for which a band filter of sufliciently sharp selectivity can be built. Equalization for attenuation and phase distortion are also more readily introduced at the lower frequencies. The band is then modulated back toits original frequency range and 4,5v

placed onthe line at the output side of the band elimination filter.

The invention may now be more fully understood by reference to the following description of an illustrative embodiment when read in connection with the accompanying drawings, in

which:

Fig. 1 is a schematic diagram of a system embodying one form of the invention;

Fig. 2 shows the attenuation characteristic of 55 ,I

a typical band elimination filter insolid line with the modifications effected in the characteristic by the shunting circuits of'Fig. 1 indicated in dotted lines;

Fig. 3 shows a typical phase characteristic of a band elimination filter;

Figs. 4 and 8 show schematic diagrams of filters to be described more in detail later; and

Figs. 5, 6, and '7 show characteristics of filters or equalizers used in the system as will be indicated lat'er.

In the illustration of Fig. 1 the iinesll and z are multiplex carrier transmission lines of a system using the well known four-wire arrangement in which one pair of conductors, shown as transmission path or line I, is used to transmit a multiplicity of carrier channels in one direction and the other pair, 2, transmits a like number of channels in the opposite direction between the terminals of the line. It is assumed in this example that the pairs are able to transmit efficiently a wide range of frequencies say from approximately 500 to 5000 kilocycles. Some types of concentric conductor pairs could be used for such a system.

It is also assumed that the pairs constituting line I and line 2 of Fig. l are of the same kind and that therefore the apparatus necessary in line I which will be described in detail will be suitable for use inline 2. Hence the block 32 of Fig. 1 can be assumed to be a duplication of theV apparatus shown in the upper portion of Fig. l and described hereunder. However it should be noted that since the direction of transmission along line 2 is assumed to be opposite to that along line I the apparatus for line 2 must be arranged from right to left in the order given for line I from left to right.

Signals transmitted along line I from the left are amplified by the line amplifier 3 whose output is connected to four fixed attenuators, 4, the input terminals of the latter being connected in parallel. These attenuators serve to prevent interaction between the filters 5, 8, 9, and I0. This is necessary where, as assumed, the theoretical transmitting ranges of these filters overlap slightly and troublesome distortion might result were the attenuators removed. In this example, these attenuators need introduce approximately 12 decibel attenuation. Assume, by way of example, that the band of frequencies between 3000 and 3200 kilocycles inclusive, is to be employed in the channels connecting to the intermediate point between the terminals in the main transmission line. Band elimination filter, 5, is employed to eliminate this band of frequencies between 3000 to 3200 kilocycles inclusive. T-he attenuation characteristic of this lter is shown in the solid line curve of Fig. 2 and its schematic diagram in Fig. 8. 'I'he function of this filter is to isolate the left portion of line I from the right portion of line I at all frequencies between 3000 to 32,00 kilocycles inclusive. However it is not possible for it to effectively do this Without introducing attenuation distortion in the adjacent frequency bands 2800 to 3000 kilocycles and 3200 to 3400v kilocycles respectively. The phase characteristic of the band elimination filter is shown in Fig. 3. Phase equalizer 6 and attenuation equalizer 1 may be used if desired to compensate more remote frequencies for phase and attenuation distortion but cannot fully compensate for the large amount of distortion of the band elimination filter in the immediately adjacent bands without destroying its effectiveness. vIn the more usual cases equalizers I-and 1 may be omitted.

However, each of the adjacent frequency bands jacent group upside down to the frequency range 300 to 500 kilocycles. These frequencies pass through the band lter I4 whose attenuation characteristic is shown by curve b of Fig. 5, and whose schematic diagram for any single section is shown in Fig. 4. As shown this is a lattice type band-pass filter employing two piezoelectric crystal elements per section. Ordinarily several sections like Fig. 4A are connected in tandem, each section contributing a portion only of the total selectivity. In the present embodiment five such sections are used in tandem. The function of lter I4 is to exclude all unwanted frequencies present in the output of demodulator II and to give sharp selectivity for the band 2800-3000 kilocycles now demodulated to 300 to 500 kilocycles. The gain in selectivity obtained by demodulating so as to use the lower frequency band filter is illustrated graphically in the difference between curves a and b of Fig. 5. The next step in the shunt circuit is to introduce the proper attenuation and phase equalization by means of equalizers I5 and II respectively whose characteristics are shown in Figs. '7 and 6 respectively. In this example the equalization is directed to combine in proper phase and amplitude frequencies between2800 and 3000 kilocycles which pass through the band elimination filter with the same frequencies which pass through this special shunting circuit to the end that the effective attenuation characteristic of the combination of the band elimination filter and the special shunting circuit will differ from that of the band elimination filter above as indicated by the dotted line shown at 3000 kilocycles and below in Fig. 2. An added advantage is that equalization is more readily introduced at the lower frequencies. After equalization the 300 to 500 kilocycle band is modulated by group modulator I1 back to the range 2800 to 3000 kilocycles.

lBy means carrier phase shifter 25 an arbitrary phase shift of the entire groupcan be introduced as necessary to assist in phase equalization. The group band filter 8 is now used to select the proper modulation product such as the lower side-band and to exclude unwanted frequencies present in the modulator output. A fixed attenuator 4 com pletes the shunting circuit and permits the connection of the output 'of the shunting circuit in vparallel with the' output of the band elimination filter circuit without danger of interaction between the two.

The composition and operation of the shunting circuit for the upper adjacent group is exactly, the same as for the lower adjacent group as just described except that the group band filter S of course selects the upper adjacent group of fre-- quencies; demodulator I2 and modulator 20 are supplied by oscillator 23 with a carrier at 3700 .kilocycles and the attenuation and phase equalizers I8 and I9 are designed for the upper adjacent group. The resulting effective modifications in obtained by means of these shunting circuits are shown by the dotted lines in Fig. 2 and the obiective of isolating the left portion of line I from'.

the right portion of line I at frequencies chosen by way of example between 3000 to 3200 kllocyclcs and at the same time permitting the free transmission o'f the other frequencies of the system .l

21 and suitable carrier receiving apparatus 29.

Transmission to the line may originate in suitable carrier transmitting apparatus 30, pass through the group amplifier 28, the modulator 2i, group l band filter I0, and a fixed attenuator I to the line amplifier 3 at the upper right side in Fig. l.

While Fig. 1 indicates the carrierreceiving and transmitting apparatus connecting'directly to the line at the intermediate point, the receiving and transmitting apparatus may be located at some more remote point and may be connected vto the intermediate point by a suitable carrier transmission line with intermediate repeaters if necessary.

. It is to be understood that the circuits shown and described are merely illustrative of a preferred form of practicing the invention but that neither the drawings nor the description should be taken as limiting the invention, the scope of which is defined in the claims.

What is claimed is:

1. The combination with a multiplex carrier current line capable of providing a large number of carrier channels of a circuit for diverting from or adding to the line a portion only 4of said channels at an intermediate point comprising bandpass filters connected to the line at said point to permit transmission from or to the line of such frequencies only as are included in the channels to be diverted from or added to the lineat said point respectively, and a band elimination lter connected in the line between the points to which the above band-pass filters are connected to effectively isolate the portions of the line oneither side of said band elimination filter fromeach other at the frequencies which arel included in the channels to be diverted from or added to the line at said point but permitting free transmission along the line over the other carrier channels, and a circuit connected between said line portions in shunt relation to said band elimination filter for the purpose of obtaining sharper selectivity between the band eliminated by said lter and a band of frequencies adjacent to the eliminated band of frequencies, said shunt circuit consisting of an input band-pass filter passing a group of frequencies a improved selectivity being included as part of the shunting circuit, suitable attenuation and phase 'equalizers designed so that frequencies passing through the shunting circuit will appear on the output of the band elimination lter circuit in proper volume and phase relation tov the same frequencies which pass through the band elimination filter circuit, a modulator for restoring the frequencies passing through the shunting circuitj to their original absolute values and a third bandv pass filter similar to the input band-pass filter to afosaaca -the characteristic of the band elimination filter l attenuate unwanted frequencies. present in the modulator output and to pass the desired frequency band from the modulator to the line at the output end of the band elimination filter circuit. 2. The combination with a multiplex-carrier current line capable of providing a large number of carrier channels of a circuit for diverting from or adding to the line a portion only of said channels at an intermediate point comprising bandpass filters connected to the line at said point to permit transmission from or to the line of such yfrequencies only as are included inthe channels to be diverted from or added to the line said point, and a band elimination filter connected in the line between the points to which the above band-pass filters are connected to effectivelyv isolate the portions of the line on either side of said band elimination filter from each other at the' frequencies which are utilized by the channels to be diverted from or added to the line'atlsaid point but permitting free transmission along the line over the other carrier channels, and a pair of circuits shunted around the circuit arrangement including said band elimination filter for the purpose'of compensating frequencies adjacent to the band eliminated by said filter for attenuation or phase distortion introduced by said band elimination filter, each said vshuntingcircut comprising an input band-pass filter passing frequencies adjacent to the eliminated band of frequencies, and

frequencies of the band elimination filter, a demodulator for the purpose of changing the absolute frequencies' of the respective adjacent band so as to make possible the use of more precise or more readily constucted band-pass filter and phase and attenuation equalizers included as part requisite degree of selectivity to select the desired frequency range after demodulation, the above described phase and attenuation equalizers to modify the adjacent frequencies in a manner to compensate'for 'd'stortion ofthe type caused by the band elimination filter in the line, a modulator to restore the frequencies to the range of their original absolute values and a -third band-pass filter vsimilar pto' the input band-passA filter to attenuate unwanted frequencies present in the modulator output and to pass the proper sideband from the modulator to the line at the output'end of the band elimination filter circuit.

3. In a line `including al four-terminal frequency selective device which distorts waves inv the region of its cut-off, a circuitin shunt relation to said vdevice including means to compensate such distortion, said means comprising a demodulator for reducing the frequency of transmitted Waves derived from said line on one side of said device including frequencies in the range in which such distortion occurs, an equalizer for counteracting the effect of vsaid distortion in the demodulated wave, and means to raise the frequency of the equalized waves to ltheir original frequency position and to impress them on said line on the other side of said device, the waves being impressed on the line to combine with the waves passed by said device to compensate the distortion therein.

4. In combination with vartour-terminal frequencyselective device having the characteristic `to produce distortion at the edge of the band of frequencies selected, circuits including a fourterminal shunting device connected to one side of said first mentioned device providing a path around the'circuit including the first mentioned .of the shunting circuit, .a band-pass filter of the mentioneddevice; said elements including means to deniodulate waves of frequencies in a band distorted by thc rst mentioned device, means for sharply discriminating against unwanted frequencies, equalizer means for correction of attenuation and phase distortion, and modulator means to restore the frequencies to their original band prior to placing thel corrected frequencies on the opposite side of the iirst mentioned selective device.

5. The combination with a four-terminal transmission device having an insuiciently sharp cutofl at the-high frequencies to be transmitted on a. multiplex carrier telephone line, of shunt circuits including a four-terminal shunting device providing a path from one'side of said iirst mentioned device around the circuits including the first mentioned device, means in the shunting device to select and modulate a portion of the frequency range distorted by the first mentioned device to a lower frequency range to make possible the use in the shunting device of equalizing devices of more precise or desired discriminating characteristics, means for filtering in the shunting device more precisely according to the characteristic desired in the rst mentioned device, equalizing means for correcting attenuation and phase of signal energy desired to be transmitted by the nrst device, and modulating means followed by lter means for placing the output of the shunt circuit on the output side of the rst mentioned transmission device.

6. The combination with a band pass lter and a line having too slow a rate of change of attenuation adjacent a band to be eliminated from the line, of four-terminal shunting devices providing paths from one side of said filter around the circuit including the filter in the line and adapted to. contribute tq the ltering action intended for the rst mentioned filter, the shunting devices having means arranged to modulate frequencies chosen inranges sought to be corrected, lters operating in the new frequency ranges having more precise characteristics and cut-off for the signals desired to be eliminated by the lter in the line, attenuation and phasev equalizers for producing materially better equalization and phase correction of the signal frequencies d'esired to be transmitted in the line through the iirst mentioned lter, modulating means to restore the signal energy to the desired frequency range, and filter means and attenuation means arranged to place the restored frequencies on the line at the output side ofthe iirst mentioned filter.

7. 'Ihe combination with a four terminal frequency selective device having an/undesired rate of attenuation change with frequency of a four terminal shunting device providing a parallel path around the circuit including the first-mentioned device and means in said parallel path of greater precision of cut-oil for correcting for the distortion arising from the sad undesired rate of attenuation change with frequency, thereby obtaining sharper selectivity between the attenuated and the transmitted'frequency ranges of the original device.

CHARLES L. WEIS, JR. 

